An optical memory technology using an optical information recording medium having a pit-shaped pattern as a high-density/large-capacity recording medium is expanding its application range, for example, digital audio disk, video disk, document file disk and, what is more, data file. According to the optical memory technology, information is recorded, reproduced or erased to/from an optical information recording medium using micro condensed light beams with high accuracy and reliability. This recording/reproducing operation or the like depends on an optical system.
Basic functions of the optical pickup apparatus which is principally composed of an optical system can be roughly divided into light condensing that forms micro spots of diffraction limits, focus control and tracking control of the optical system and detection of pit signals. These functions are realized by combinations of various types of optical systems and photoelectrical conversion/detection systems according to their purposes and applications. In recent years, a diffraction element (hologram) is used to reduce the size and thickness of the optical pickup apparatus (e.g., see Japanese Patent Laid-Open No. 2001-176119).
This type of optical pickup apparatus is shown in FIG. 5. Patent Document 1 describes a case where two wavelengths are used, but an apparatus using three wavelengths will be shown here and operations thereof will be explained.
A light source 101A is a light source which emits light having a first wavelength and a light source 101B is a light source which emits light having a second wavelength which is longer than the first wavelength and light having a third wavelength which is longer than the second wavelength.
Light emitted from the light source 101A is reflected by an optical path coupling member 104, condensed by a light condensing member 105, irradiated onto an optical information recording medium 106 and reflected light reflected by the optical information recording medium 106 is condensed by the light condensing member 105, passes through the optical path coupling member 104, enters a diffraction element 103 where it is diffracted so as to selectively enter predetermined photodetectors 102 (102A to 102D).
Light emitted from the light source 101B is condensed by the light condensing member 105, irradiated onto the optical information recording medium 106, reflected light reflected by the optical information recording medium 106 is condensed by the light condensing member 105, enters the diffraction element 103 where it is diffracted so as to selectively enter the predetermined photodetectors 102 (102A to 102D).
That is, if the light diffracted toward the right in the figure by the diffraction element 103 is defined as − (minus) and the light diffracted toward the left is defined as + (plus), of the diffracted light of the first to third wavelengths, +1st order diffracted light is input to the same photodetector 102A, and with regard to −1st order diffracted light, diffracted light of the first wavelength light enters the photodetector 102B and diffracted light of the second wavelength light enters the photodetector 102C and diffracted light of the third wavelength light enters the photodetector 102D. An electric signal is then output according to the amount of light received from the photodetectors 102A to 102D.